Mesh-reinforced secondary electron conduction target for camera tubes

ABSTRACT

A target structure is disclosed having superior electrical properties and mechanical strength. The target comprises a layer of KC1 supported by a dielectric material which in turn is deposited on a fine nickel metal mesh which provides mechanical support for the target. This structure is particularly useful in the construction of secondary electron conduction (SEC) targets.

United States Patent Inventor Appl. No.

Filed Patented Assignee MESH-REINFORCED SECONDARY ELECTRON CONDUCTION TARGET FOR CAMERA TUBES 2 Claims, 3 Drawing Figs.

11.8. C1. 313/89; 313/65; 313/67; 313/311; 315/11 Int. Cl ..ll0lj 31/26, H01j3l/30,l-l01j29/45 Field of Search 313/65'1, 65, 68, 89

Primary Examiner.lohn Kominski Assistant ExaminerV. Lafranchi Anorneys-Nathan J. Cornfeld, John P. Taylor, Frank L. Neuhauser, Joseph B. Forman and Oscar B. Waddell ABSTRACT: A target structure is: disclosed having superior electrical properties and mechanical strength. The target comprises a layer of KC 1 supported by a dielectric material which in turn is deposited on a fine nickel :metal mesh which provides mechanical support for the target. This structure is particularly useful in the construction of secondary electron conduction (SEC) targets.

s-s-s.

INVENTOR: IDEAL 1r. SALDI, W

Hl'S ATTORNEY.

ATENTEU FEB] 6 1971 MESH-REINFORCED SECONDARY ELECTRON CONDUCTION TARGET FOR CAMERA TUBES BACKGROUND OF THE INVENTION This invention relates to improvements in target structures for camera tubes. In one aspect the invention relates to an improved secondary electron conduction target. The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.

Targets for camera tubes conventionally comprise a thin membranelike layer of active material carried in drumhead fashion on a supporting ring. Due to the delicate physical nature of some active materials a supporting layer is sometimes desirable to physically support the target. The support layer, when used, usually comprises a thin, nonactive material selected so as to not interfere with the passage of photons or electrons to the active material. Furthermore, the support layer comprises a conducting portion, for example, a thin metallic layer on the photocathode side of the support layer.

However, when the supporting layer is made thick enough to provide the intended physical for the target, it has been found that the energy requirements to pass light or an electron beam through the support layer, including the metallic portion of the support layer, to the active target material are greatly increased.

It has now been discovered that active target material can be physically supported by means having enhanced transparency to electrons or light, thus mitigating the energy requirements heretofore necessary.

SUMMARY OF THE INVENTION In accordance with the invention, an improved structure for an image device is provided wherein means having a high transparency to electrons are provided to physically support a layer of active material. In one aspect the invention relates to a target structure wherein the target cm comprises a secondary electron conduction (SEC) material supported by a dielectric layer which is carried by a finely divided nickel metal mesh providing both physical support for the target and acting as the collection electrode.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an axial view in cross section, schematically representing a camera tube to which the present invention may be applied.

FIG. 2 is an enlarged, partially cutaway, isometric view of the invention.

FIG. 3 is a cross-sectional view of a portion of FIG. 2 taken along lines 2-2.

DETAILED DESCRIPTION Referring now to FIG. 1, a camera tube is generally indicated at 2 having an outer glass envelope 10, an electron optics section 20, a photocathode 30, and target section 40. Electron optics section 20, comprises a cathode 22 and suitable electrodes and coils for the collimation, acceleration, focus and deflection of an electron beam emitted from the cathode. Electron optics useful in the practice of the present invention can be found in US. Pat. No. 3,3 19,1 10, issued May 9, I967 and assigned to the assignee of this invention. Suitable external circuitry (not shown) controls the scanning or sweeping of the beam.

Photocathode 30 comprises a layer 32 of photoemissive materialwhich can be coated on the inner face of a window 12 formed on the end of envelope l0. Electrons emitted from the photocathode are, in response to light imaged thereon, accelerated by electrode 34 toward target section 1 40.

Referring now to FIGS. 2 & 3, in accordance with the invention target section 40 generally comprises an metal support ring 42 upon which is tautly mounted a finely divided metal mesh 44. Metal mesh 44 is preferably constructed of nickel LII which does not chemically react with the target materials including the dielectric layer to bemounted directly thereon and does not oxidize under conditions used to oxidize the dielectric material. Nickel is also preferred because it does not warp under subsequent heating conditions, as does, for example, copper which could rupture the target materials mounted thereon. As will be noted below the metal mesh, when constructed on nickel, can actually be tightened as well as cleaned, by baking in a reducing atmosphere subsequent to formation of the target structure. Metall mesh 44 is preferably of 1000 line per inch or finer gauge with a transverse thickness of about 0.2 mils and a ratio of mesh to opening area sufficient to provide about 60 percent transmission of light. Such meshes are commercially available or can be constructed by electrodeposition of the metal on ruled glass.

Mesh 44 in turn carries a thin dielectric layer 46 of magnesium oxide. Mesh 44 and layer 46 together form a target supporting structure for a layer of secondary electron conduction (SEC) material such as KCl which comprises the active material of the target.

To construct target section 40, metallic mesh 44 is peripherally welded to edge 50 of support ring 42. A thin film of a removable material is then applied to the outer face of the mesh. This can be accomplished, for example, by dropping onto the surface of a pan of water, a small quantity of nitrocellulose dissolved in an organic solvent such as amyl acetate. This solution spreads out into a thin film because of the surface tension and the solvent evaporates, leaving a plastic film. The mesh which is placed in the water, either prior to forma' tion of the film or which is immersed into the water around the outside of the film is then raised gently to pick up the film on the top surface of the mesh.

After the film has dried completely, the mesh is placed in an evaporator and a thin coating of magnesium is evaporated onto the film. It is desirable to constnict the dielectric layer thick enough to provide support for the active target mate rial, yet thin enough to not excessively interfere with the passage of electrons therethrough from the photocathode. In a preferred embodiment of the invention, a layer of magnesium sufficient to provide a 50 I00 angstrom layer of MgO upon subsequent oxidation is evaporated onto the film.

The target assembly is then baked in an oxidizing at mosphere at a temperature of at least 400 C for about 5 hours to vaporize the plastic film and to oxidize the magnesium, converting it to magnesium oxide. The target assembly is then baked in a reducing atmosphere at about 800 C which tightens and cleans the mesh.

A low density layer 60 of secondary electron conduction (SEC) material, preferably potassium chloride, is then evaporated onto dielectric support layer 46 and metal mesh 44 forming the structure illustrated in FIGS. 2 & 3. In the preferred embodiment the SEC material comprises a 2030 micron layer of potassium chloride evaporated onto the supporting materials in an inert atmosphere such as argon at a pressure of about 2 millimeters of mercury. As best shown in FIG. 3, the layer of KCl is evaporated onto the opposite side of the mesh with respect to the dielectric layer thus making the mesh the core of the target structure.

The novel SEC target of the invention is then conventionally mounted in camera tube 2 which is operated in the direct beam vidicon type mode. In operation, an electric field is established across the target by applying a positive potential to metal mesh 44 which acts as the signal plate and stabilizing the opposite surface of the potassium chloride layer at ground or reading gun cathode potential. This is done by means of a low velocity electron scanning beam.

An image focused on the photocathode results in a pattern of electron emission proportional to the light intensity from point-to-point along the optical image. This electron image impinges on the target with an energy of 8 kev. Most of this energy is absorbed in the KCl layer, resulting in many free secondary electrons. Under the influence of the electric field applied across the target, initially most of these secondary electrons are collected by the signal plate, causing the KCl layer to charge to more positive potentials. This process continues until the exit surface of the [(01 is charged to the signal plate potential. Beyond this point, the electric field across the target reverses polarity, resulting in a reversal of direction in conduction current. Due to the increasing effect of transmitted secondary emission, the KC! layer is charged to even more positive potentials. Scanning of the beam across the target surface restores the target surface to ground potential, thereby giving rise to a current pulse in the signal plate. This current pulse is passed through an external resistor to develop a video signal.

Camera tubes using the target structure of the invention exhibit low lag and good antihalation properties. In this regard it has been noted that secondary electron redistribution resulting in black border halation appears to be eliminated by using the novel target structure of the invention. While the exact reasons are not known it appears that the dielectric layer of magnesium oxide inhibits redistribution of the secondary electrons on the target material. The energy requiqememts for acceleration of the photoclthode-ernitsed electrons to the target aqe much lower since the photoelectrons must only penetrate the MgO film (requiring about 500 volts) of the target structure in comparison with prior art target structures. In addition, the novel SEC target structure of the invention provides increased resolution. Furthermore, camera tubes using the target structure of the invention can be subject to a more extended environmental range of service due to the ruggedness of the target structure.

It should be pointed out in this regard that the invention may be further modified by the addition of thin conducting layers for modified electrical properties; such layers need only be thick enough to provide necessary conduction, there being no necessity to provide further physical strength to the target as in prior art constructions. These and other such minor modifications are deemed to be within the scope of the invention as defined by the appended claims.

lclaim:

l. A target structure for a scanning beam electronic device comprising a metallic support frame member, a finely divided nickel metal mesh rigidly positioned across said frame member, a 50-100 angstrom layer of magnesium oxide carried on one side of said metallic mesh, and a layer of secondary electron conduction material on the opposite side of said mesh, said layer of secondary electron conduction material, being physically supported by said mesh and said magnesium oxide layer.

2. The target structure of claim 1 wherein said layer of secondary electron conduction material comprises a 2030 micron layer of potassium chloride deposited thereon. 

2. The target structure of claim 1 wherein said layer of secondary electron conduction material comprises a 20-30 micron layer of potassium chloride deposited thereon. 